In a typical tool for measuring the shape and thickness of a silicon wafer, two channels of interferometers are employed to measure both surfaces of the wafer. Each interferometer usually comprises lenses that image the wafer to a video camera. This way the whole wafer can be measured by the camera with millions of pixels, eliminating the need to mechanically scan the wafer, and the throughput is dramatically improved compared to scanning systems.
One disadvantage of this method is that the size of the measuring tool is large due to the size of imaging optics. As the semiconductor industry shifts to larger wafers (for example from 300 mm to 450 mm) the size of the measuring tool may increase significantly. Simply scaling up existing measuring tools designed for a 300 mm wafer to accommodate a 450 mm wafer would result in a measuring tool much more expensive and fifty percent larger in every direction. At that size, the measuring tool may not physically fit in a space currently designated for such measuring tools.
Consequently, it would be advantageous if an apparatus existed that is suitable for measuring the shape and thickness of a silicon wafer with a compact optical arrangement.